A Hypothesis for the Rapid Formation of Planets

Abstract

The discovery of 1I/'Oumuamua confirmed that planetesimals must exist in great numbers in interstellar space. Originally generated during planet formation, they are scattered from their original systems and subsequently drift through interstellar space. As a consequence they should seed molecular clouds with at least hundred-metre-scale objects. We consider how the galactic background density of planetesimals, enriched from successive generations of star and system formation, can be incorporated into forming stellar systems. We find that at minimum of the order of 10 7 'Oumuamua-sized and larger objects, plausibly including hundred-kilometre-scale objects, should be present in protoplanetary disks. At such initial sizes, the growth process of these seed planetesimals in the initial gas-and dust-rich protoplanetary disks is likely to be substantially accelerated. This could resolve the tension between accretionary timescales and the observed youth of fully-fledged planetary systems. Our results strongly advocate that the population of interstellar planetesimals should be taken into account in future studies of planet formation. As not only the Galaxy's stellar metallicity increased over time but also the density of interstellar objects, we hypothesize that this enriched seeding accelerates and enhances planetary formation after the first couple of generations of planetary systems.1 Here the term 'ISOs' describes free-floating planetesimals with a size 50 m.

2MASS J15460752−6258042: a mid-M dwarf hosting a prolonged accretion disc

“…Recently, several examples of circumstellar disc accretion at ages greater than ∼10 Myr have been identified (e.g., PDS 66-20 Myr; Mamajek et al 2002, HD 21197-30 Myr;Moór et al 2011, 49 Ceti-40 Myr;Zuckerman & Song 2012, WISE J080822.18-644357.3-45 Myr;Silverberg et al 2016;Murphy et al 2018, J0446A & B, J0949A & B;, and they have been treated as unusual anomalies without delving into the problem of prolonged gas accretion at extreme ages. In spite of the small number of cases, such E-mail: song@uga.edu old pre-MS stars hosting accretion discs can be challenging to the hypothesis for the rapid planet formation (Pfalzner & Bannister 2019;Manara et al 2018;Najita & Kenyon 2014;Greaves & Rice 2010) and provide the upper limit on the life times of gas-rich discs.…”

Detecting Interstellar Objects through Stellar Occultations

“…Here, we propose an analagous search for interstellar objects (ISOs), flagged by their unusual inclinations and unique kinematics leading a distribution of characteristic distribution of durations, the peak of which lies between that of Kuiper belt objects and that of Oort cloud objects. amir.siraj@cfa.harvard.edu, aloeb@cfa.harvard.edu The data from such a search would calibrate population parameters for ISOs crucial for constraining formation theories of exoplanetary systems (Duncan et al 1987;Charnoz & Morbidelli 2003;Veras et al 2011Veras et al , 2014Pfalzner et al 2015;Do et al 2018;Raymond et al 2018;Hands et al 2019;Pfalzner & Bannister 2019;Siraj & Loeb 2019d). Below we derive the physical characteristics and rate of the expected ISO occultation events.…”

In-Situ Exploration of Objects on Oort Cloud Comet Orbits: OCCs, Manxes and ISOs

“…LPCs come from the Oort Cloud (OC)-a swarm of relict planetesimals located between ∼5,000-100,000 au, emplaced during solar system formation-gravitationally scattered outward by the giant planets (mainly Jupiter; (Duncan et al, 1987;Dones et al, 2015). This happened after the Sun left its birth cluster (Tremaine, 1993;Pfalzner & Bannister, 2019). Some fraction of the Oort cloud could have been captured from other stars in the Sun's birth cluster, if most stars scatter out a lot of planetesimals (Levison et al, 2010).…”